1. Field of the Invention
The present invention relates to drill bits used to create boreholes through a material and, more particularly, to such drill bits that are used in the exploration and production of hydrocarbons.
2. Setting of the Invention
In the exploration and production of hydrocarbons, a rotating drill bit is used to create a borehole through the earth's subsurface formations. The users of the drill bits and the drill bit manufacturers have found that by controlling more precisely the weight-on-bit (WOB) and increasing the rotational speed (RPM) that increased penetration rates can be achieved. However, as the RPM is increased, the drill bit effective life has decreased dramatically because the cutting elements on the drill bit become cracked and occasionally are violently torn from the bit body.
Numerous studies have been made to find out what causes such destruction to the cutting elements. The inventors hereof have previously found that a substantial portion of the destructive forces are generated by radial imbalance forces that cause the drill bit to rotate about a center offset from the geometric center of the bit body in such a way that the drill bit tends to backwards whirl about the borehole. This whirling causes the center of rotation to change dynamically as the drill bit rotates about the borehole. Thus, the cutters travel faster, side-ways and backwards and are subjected to greatly increased impact loads, thereby destroying the cutters.
More specifically, circumferential drilling imbalance forces exist to some degree on every drill bit and these forces tend to push the drill bit towards the side of the borehole. If the drill bit has a normal cutting structure, the gauge cutters are designed to cut the edge of the borehole. During the cutting process, the effective friction between the cutters near the gauge area increase and, thus, the instantaneous center of rotation becomes some point other than the geometric center of the drill bit. When this happens, the usual result is for the drill bit to begin to backwards whirl around the borehole. The whirling process regenerates itself because sufficient friction is always generated between the drill bit gauge area and the borehole wall--no matter what the orientation of the drill bit--from the centrifugal forces generated by the rapid acceleration of the drill bit.
Various methods and equipment have been proposed to eliminate or reduce these imbalance forces, including using dynamically balanced lower drillstring assemblies and realigning the cutters to reduce the imbalance forces.